The present invention is directed to a method of operation for a direct injection gasoline engine including a fuel vapor purge system, and more particularly to a method of controlling the engine combustion mode and fuel supply based on an estimate of the hydrocarbon concentration of the purge vapor.
A direct injection gasoline engine may be operated in either homogeneous or stratified combustion modes, depending on the fuel injection timing relative to the engine cycle. In the homogeneous combustion mode, fuel is injected during the intake stroke of a four-stroke cycle so that the air/fuel mixture is evenly distributed throughout the cylinder when the mixture is ignited during the combustion stroke; while operating in this mode, a closed-loop control of fuel is executed to maintain the air/fuel ratio at a desired value, such as the stoichiometric ratio. In the stratified combustion mode, fuel is injected during the combustion stroke, resulting in a rich air/fuel mixture in the vicinity of the spark plug at ignition, even though the overall air/fuel ratio in the cylinder may be significantly leaner than the stoichiometric ratio; while operating in this mode, closed-loop fuel control is suspended, and an open-loop fuel control is executed instead.
Storage of fuel tank vapors is essential for the control of evaporative emissions, and in the usual system, the stored fuel vapor is periodically purged into the intake manifold of the engine for delivery into the engine cylinders along with the intake air. However, the purged fuel vapor may constitute a significant percentage of the overall fuel requirement, and the fuel injection quantity must be adjusted accordingly to maintain accurate control of the air/fuel ratio. When an engine is operated in a homogeneous combustion mode engine with closed-loop fuel control, the hydrocarbon concentration of the purge fuel vapor may be estimated during the periodic purging based on the feedback signal of the closed-loop exhaust gas oxygen sensor, and then used to suitably adjust the fuel injection quantity; see for example, the co-pending U.S. patent application Ser. No. 09/264,524, (Attorney Docket No. H-203439), filed on Mar. 8, 1999, assigned to the assignee of the present invention, and incorporated by reference herein. However, the disclosed approach cannot be used to estimate the purge vapor concentration during open-loop fuel control in a stratified combustion mode. Also, it is difficult to effectively purge stored fuel vapor while operating in the stratified combustion mode because the intake manifold pressure is significantly increased during stratified combustion, and the vapor that is successfully purged tends to burn incompletely. Accordingly, what is needed is a control method for a direct injection gasoline engine that regulates engine operation to efficiently purge stored fuel vapors while maintaining accurate fuel injection control during purging.
The present invention is directed to an improved control method for a direct injection gasoline engine operable in stratified or homogenous combustion modes and having a fuel vapor purge system, wherein the hydrocarbon concentration of purge vapor is estimated during open loop fuel control in the stratified combustion mode, and wherein the fuel injection quantity and the combustion mode are controlled based on the estimated concentration. The hydrocarbon concentration of the purge vapor is estimated during open-loop fuel control by measuring the air/fuel ratio error during steady state operation with no fuel vapor purging, and using the measured steady state air/fuel ratio error to normalize the air/fuel ratio error observed during steady state operation with purge control. The fuel injection quantity is compensated for the estimated purge vapor concentration, and engine combustion mode is determined in part based on a comparison of the estimated concentration with a calibrated threshold. Additionally, a limit on the percentage of purge vapor is determined as a function of combustion mode, and the flow rate is controlled based on the degree to which the percentage of purge vapor exceeds the determined limit.